1. Field Of The Invention
The present invention relates in general to a mineral mining installation and, more particularly, to a propulsion system for a mineral winning machine which is moved back and forth along a mineral face.
2. Description Of Prior Art
Reactive-track type propulsion systems for moving a winning machine, usually a shearer, along a mineral face are known in which a toothed sprocket wheel on the machine is rotated and engages or meshes with a track fixed along the path of movement of the machine. The track is composed of sections mounted onto the pans of a scraper-chain conveyor. It is also known to employ a sprocket chain as the track. In known arrangements described, for example, in French Patent Specification No. 2 523 639 and in German Patent Specifications Nos. 2 600 536 and 2 938 408, the teeth of the drive wheel located in horizontal links of the chain while vertical links of the chain engage between the teeth so that the wheel rolls over these links and the reactive force propels the winning machine. The chain is located in a carrier or guide which traps the horizontal links but allows movement longitudinally of the chain. Normally, a guide tongue on the shearer engages beneath a flange or strip on the carrier to hold the machine down and prevent the drive wheel lifting clear of the chain. In another known propulsion system, described in U.S. Pat. No. 4,372,619, an oval link chain is used as the track and the vertical links of the chain are shorter than the horizontal links and provide minimal clearance for the horizontal links thereby restricting the tensional and contractional play in the chain and ensuring the correct pitch for the teeth of the drive wheel. The inherent articulation between the links permits the chain to flex however.
A disadvantage of the known systems is that even slight variations in the distance between the longitudinal axis of the chain and the rotational axis of the drive wheel create considerable transverse forces up to 15% to 20% of the propulsive force. During operation, the forces produced by the cutting drums of the shearer are considerable and often tend to lift the machine. This is resisted by the components of the hold-down means which slide relative to one another with high friction. The lifting of the machine however causes variation in the distance between the chain axis and the axis of the drive wheel and transverse forces are exerted on the teeth of the drive wheel to aggravate the situation and further increase the friction between the components of the hold-down means. Rapid wear of the components then results which reduces efficiency.
The use of a chain of the type described in U.S. Pat. No. 4,372,619 as the track for the drive wheel requires a relatively large pitch for the drive wheel teeth and to restrict the size of the drive wheel it is necessary to have the lowest possible number of teeth. This necessitates heavy undercutting of the roots of the teeth and this makes the wheel more liable to bend under stress. To preclude this, additional reinforcement is provided but this tends to modify the operative shape of the teeth and tooth contact occurs mostly above the longitudinal axis of the chain. This again creates lateral forces and the effects discussed above.
A general object of the present invention is to provide an improved propulsion system.